Sir David Attenborough has said Planet Earth II is attracting viewers in their millions because it provides "a form of two-way therapy" at a time when people have grave concerns for the world.

The BBC natural history series is pulling in more than 12 million viewers a week and is on course to become the second most-watched TV programme of the year, after The Great British Bake Off.

Sir David said the programme is tapping into the public's need for sanctuary, providing a refuge in the way his show Blue Planet did when it aired in 2001 after the September 11 terrorist attacks in America.

Planet Earth II (BBC)

He told Radio Times: "The viewing figures for Blue Planet were way beyond our expectations.

"Yes, the series was beautifully filmed, but there was something much more significant going on. As a nation we craved refuge from the horror and uncertainty, and for an hour on Wednesday evenings our oceans provided that sanctuary."

He said Planet Earth II is "tapping into a similar sentiment".

"Of course, there is no single appalling catalyst as there was in 2001, but our concerns for the world, the confusion we have about its direction of travel, are every bit as great," said Sir David.

"The planet has rarely felt in greater environmental peril."

Planet Earth II (BBC screenshot)

The broadcaster, 90, said the people who tune into Planet Earth II are "reconnecting with a planet whose beauty is blemished, whose health is failing" because they understand that our own wellbeing is connected to that of the planet.

"It's a form of two-way therapy," he added.

Other factors in the show's success, he said, include the proximity to the animals due to technological advances, and the fact that the programme airs in the 8pm Sunday evening slot.

"I'm told that we are attracting a larger than normal number of younger viewers and apparently the music of Hans Zimmer in particular is striking a chord with the young," he said. "And that pleases me enormously."